Time-of-flight method measurement refers to the use of a pair of beam transducers to alternately (or simultaneously) emit and receive beams toward each other, the flow rate of measured fluid is calculated by detecting the time difference between forward and reverse flow propagation of the beams in a medium, and then flow quantity is calculated by the flow rate. The beams which can be applied to flow measurement include acoustic waves, ultrasonic waves and light waves.
Beam flowmeter has been applied in more and more occasions because it has no moving parts and is long in service life, high in accuracy and good in stability. For example, two-channel and multi-channel ultrasonic flowmeters have been gradually applied to fields with high accuracy requirements such as petrochemical industry. By the use of an on-site plug-in sensor, a large-caliber beam flowmeter can be equipped with a flow measuring device on an existing pipeline, thus greatly reducing the cost of pipe section processing, transportation, on-site installation and fluid transportation interruption of the large-caliber flowmeter.
Two-channel or multi-channel beam measurement can greatly improve the stability and accuracy of flow measurement. This is mainly because during single-channel measurement, there is only one beam path which passes through the center of a pipeline. From theoretical analysis and practical verification, it has been confirmed that the flow measurement coefficient of the measurement method, namely the ratio of a time difference measurement result to actual flow is greatly influenced by Reynolds number. The Reynolds number of the fluid is related to the flow rate and viscosity of the fluid and the caliber of the pipeline. Even if a plurality of channels passing through the center of the pipeline are used, the influence of flow rate distribution on the flow measurement coefficient cannot be solved. Therefore, the stability and accuracy of single-channel flow measurement or multi-channel measurement with all channels passing through the center of the pipeline cannot meet the requirements of most industrial applications.
Taking an ultrasonic flowmeter as an example, at present, most plug-in beam flowmeter sensors can only measure the sound channel passing through the axis of a pipeline. The plug-in beam sensor is presented with a certain angle between the emission direction and the installation axis thereof. Although the installation process of the sensor is simple, the measurement coefficient is greatly influenced by the flow rate and viscosity of fluid as well as turning, valves and other disturbances. By using two or more channels which do not pass through the center of the pipeline, the calculation problem of the flow coefficient can be solved, so that the flowmeter can have stable measurement accuracy under various working conditions. In particular, in the case of a path design where the chord-center has a distance to pipe center ½ of the pipe internal radius (hereinafter referred to as the “half radius chord path”), the flow coefficient is close to 1.00 in a wide range of Reynolds numbers. As shown in FIG. 5, comparing the relationship between the flow measurement coefficients of two beam paths and the Reynolds number, it can be seen that the flow coefficient of the half radius chord path is very stable under a large range of fluid conditions, which is very advantageous to the stability of flow measurement accuracy.
However, in practical applications, a beam path which does not pass through the center of a pipeline is generally only suitable for the production of pipe section type flowmeters of medium and small caliber, and installation and adjustment are conducted in the factory. It is very difficult to accurately locate and directionally install a sensor on a pipeline under the condition that a large-caliber flowmeter is produced or a hot tapping insertion flowmeter needs to be installed on site. For example, the German company Siemens has introduced a two-channel hot tapping insertion sensor for on-site installation, realizing a half chord-center distance beam path which can achieve a high measurement accuracy in a large flow range. However, the on-site installation process of the two-channel plug-in sensor is very complicated, as shown in FIG. 3 and FIG. 4, a special bevel sleeve needs to be welded on the surface of an on-site pipeline to install the sensor. The bevel sleeve has different models according to the size of the pipeline, and special tools are needed on site for bevel welding and drilling. This requires high operation precision, increases the installation difficulty and results in high installation cost, thus greatly limits its application. Besides, since the sensor is installed at an inclined angle, it is difficult to replace the sensor online without stopping the flow in the case of sensor damage during use.